Role of the CD4 Co-receptor in Reactivity to Superantigens and in negative selection: T cell reactivity to conventional peptide antigens presented by major histocompatibility complex (MHC) class I or class II molecules requires coengagement of CD8 or CD4 respectively, with the T cell receptor (TCR). The involvement of the CD8 and CD4 co-receptors in the recognition of superantigens, either by peripheral T cells or by differentiating thymocytes, is much more controversial. The presentation of mouse mammary tumor virus (MMTV) derived superantigens, as well as bacterial enterotoxins, by MHC class II, but not class I molecules has been demonstrated. Despite this pattern of presentation, superantigens responsive CD8+ CD4- T cells have been widely reported. Furthermore, in vivo administration of antibody to CD4 inhibits the deletion caused by MMTV encoded superantigens but does not affect deletion induced by the neonatal injection of staphylococcal enterotoxin B. We will examine the role of CD4 in superantigen recognition by T cells by determining the reactivity of MMTV and enterotoxin specific T cells with antigen presenting cells expressing mutant I-E molecules which do not interact with CD4 (Aim 1). We will also examine the role of CD4 in negative selection (in the thymus) by deriving mice transgenic for the CD4 non-binding Ebeta/k molecule and analyze the extent of deletion of Vbeta 6, 7, 8.1 and 9 bearing cells in response to Mtv-7 and the deletion of Vbeta 5 and 11 bearing cells in response to Mtv-9 encoded superantigens (Aim 2a). Similarly, we will determine whether the inability of class II molecules to bind CD4 affects the ability of neonatally injected of superantigens to delete peripheral T cells expressing the appropriate Vbeta gene (Aim 2b). Finally we will determine whether the involvement of CD4 as a coreceptor facilitates positive selection (Aim 2c). Using this approach, we will be able to determine the role of CD4 in the response of T cells to both endogenous and exogenous superantigens. We will also be compare the contribution of CD4 in superantigen recognition by peripheral T cells and developing thymocytes.